Regarding the question of "gaps," we have to bear in mind that it is part of the evolutionary theory that the active processes of evolution have very largely ceased, that existing forms are but a surviving remnant with enormous gaps, and that the survivors are so fitted at present to their surroundings that evolutionary forces are causative of equilibrium rather than change. We have already seen, too, that in the struggle for existence it is among the closely-allied species that the contest is more strenuous, and that the weakest, or least fitted to survive, has to go to the wall--to be wiped out. Thus it is that there is a tendency for species to become extinct, and for the gaps to be widened. The extraordinary thing is not that we have so little direct evidence of descent, but that we have so much. That there are not more links missing is due principally to the discovery of fossil remains. When an animal dies, the probabilities are, of course, enormously against geological preservation of its bones, yet the gaps are continually being filled up by geological finds, and, though the remaining gaps may be great, they are not unaccountable. I must now pass on to the remaining set of proofs of our origin. THE TALE TOLD BY THE USELESS RUDIMENTARY ORGANS. Perhaps nothing furnishes a more conclusive proof of our animal origin than the study of rudimentary structures--muscles, sense-organs, hair, bones, reproductive organs, etc. There are some which are "either absolutely useless, such as the mammæ of the male quadrupeds or the incisor teeth of ruminants which never cut through the gums; or they are of such slight service to their present possessors that we cannot suppose that they were developed under the conditions which now exist." [173] Of useless rudimentary organs, or parts of organs, there are not less than one hundred and seven in man. [174] To this category belong the coccyx--the vestige of a tail--the muscles of the ear, the vermiform appendage, etc. "The os coccyx in man, though functionless as a tail, plainly represents this part in other vertebrate animals. At an early embryonic period it is free, and, as we have seen, projects beyond the lower extremities." [175] It sometimes happens that we find external relics of a tail. Professor Haeckel, in Fig. 195, vol. i. of The Evolution of Man (library edition), shows the tail of a six months' old boy, which Granville Harrison removed by operation. The anthropoid ape, like man, has only the rudiment of a tail.